A) Field of the Invention
The present invention relates to a process for the impregnation of a metal product with a surface comprising a hard wear-resistant material.
B) Wear-Resistant Surface Layers in Iron
A wide variety of techniques are known for the impregnation of metals, e.g., iron, with a hard wear-resistant surface. Such techniques include flame spray coating and plasma spray coating. However, each of these spray coating techniques suffer from problems associated with the spalling of surface layers during the coating process and during service as well as the particularly large expense associated with the use of this technique.
Cast-in-carbides are also known in which carbide particulates are placed in a mold and molten iron is then cast. See, for example, the discussion within U.S. Pat. No. 4,119,459 to Eckmar et al. It is difficult, however, with such castings to accurately maintain the carbide particles in the desired location and in a regular distribution pattern during casting.
In addition, certain cast-on hard surfacing techniques for use with polystyrene patterns are also known in the art. See, for example, the discussion in Hansen et al., "Application of Cast-On Ferrochrome-Based Hard Surfacing to Polystyrene Pattern Castings," Bureau of Mines Report of Investigations 8942, U.S. Department of the Interior, 1985.
However, this process suffers from problems associated with the low reliability of the bond formed between the wear-resistant layer, e.g., tungsten carbide, and the foam pattern. Also when thick powder layers are used, the iron may not penetrate the layer before the iron solidifies. Due to these reasons, instead of impregnating the iron, the carbide spalls off the product.
The inventor of the present invention has also been involved in inventing other processes in an attempt to more effectively impregnate the surface of a metal, e.g., iron, with hard phases during the casting process. For example, attention is directed toward U.S. Pat. No. 5,027,878 to Revankar et al which relates to the carbide impregnation of cast iron using evaporative pattern castings (EPC) as well as U.S. Pat. Nos. 5,190,091 and 5,190,092, which relate to the impregnation of cast iron and aluminum alloy castings with carbides using sand cores.
However, despite their effectiveness, these methods also have certain drawbacks. For example, the EPC method may involve the installation of special equipment in a conventional foundry. Furthermore, castings produced by this process can suffer from distortion due to the distortion of the plastic foam replicas. On the other hand, the above sand core methods of casting carbides can involve the preparation of carbide spheres which adds to the cost of the process. The cost can be further increased if a substantially flat wear-resistant surface is desired because in such a case, a surface layer equal in thickness to approximately half the sphere diameter will need to be machined off.
Accordingly, the need still exists for a method of impregnating metal surfaces, and in particular iron surfaces with a hard wear-resistant material which is capable of overcoming the problems associated with known techniques.